I don't fucking know, but if I had to think about it... It depends on how you space them out on the membrane that is producing them. Like each frequency is taking up a certain amount of space, and as it vibrates it will eventually collide and combine with the closest frequency which will morph into a middle value between those two frequencies.
The more frequencies you leave out to begin with, the more space each frequency will have and the longer they will last. So you basically have a spectrum between hearing mud or hearing tiny fractal crystals shattering into sand. You'll have to turn knobs to choose between having more or less frequencies, and which frequencies to keep, to figure out how many you can hear simultaneously, there will eventually be a range where you're not sure if there's a difference like when you take an eye test.
I once had a bad mushroom trip where I could hear every Mariah Carey song in my head all at the same time, it felt like she was pissed at me for existing.